1. Field of the Invention
This invention relates to certain novel modified phenolic resins. More particularly, this invention relates to such resins which have improved properties.
2. Prior Art
Phenolic resins are a class of synthetic materials that have grown continuously in terms of volume and applications for over several decades. The building blocks used in greatest volume are phenol and formaldehyde. Other important phenolic starting materials are the alkyl-substituted phenols, including cresols, xylenols, p-tert-butyl-phenol,p-phenylphenol, and nonylphenol. Diphenols, e.g., resorcinol (1,3-benzenediol) and bisphenol-A [bis-A or 2,2-bis(4-hydroxylphenyl)propane], are employed in smaller quantities for applications requiring special properties. In addition to formaldehyde, acetaldehyde or furfuraldehyde sometimes are employed but in much smaller quantities. The greater latitude in molecular structure, which is provided by varying the raw materials, chemistry, and manufacturing process, has made possible an extremely large number of applications for these products as a results of the array of physical properties that arise from the synthetic options.
The early investigation of the reaction of phenol and formaldehyde began with the work of von Baeyer and others in the early 1870s as an extension of phenolbased dye chemistry. The initial experiments resulted in soluble, amorphous products whose properties elicited little interest. Insoluble, cross-linked products were reported in the late 1880s, but these products also were not perceived us useful materials. In 1899, the first patent for a phenolic-resin product intended for use as a hard-rubber substitute was granted. The first commerical product was introduced as a shellac substitute by the Louis Bluner Company in the early 1900's. Process patents were issued in 1894 and 1895 for ortho- and para-methylolphenol, respectively.
Key innovations in early phenolic-resin manufacture included control of the molecular structure and the use of heat and pressure to achieve desirable physical properties in filled compositions. Studies in the use of acidic or basic catalysts and of changes in the molar ratio of formaldehyde to phenol resulted in the definition of two classes of polymeric materials which are referred to as Bakelite resins. Caustic-catalyzed products, which are prepared with greater than a 1:1 mol ratio of formaldehyde to phenol, can be used to form cross-linked, insoluble, and infusible compositions in a controlled fashion. With less than a 1:1 mol ratio of formaldehyde to phenol, the resultant products remain soluble; furthermore, acid catalysis yields permanently stable compositions, whereas base-catalyzed materials can be advanced in molecular weight and viscosity. Possibly of greatest importance to early commercialization, however, was the reduction to practice of the use of heat and pressure to produce essentially void-free molding compositions.
Resole resins are made with an alkaline catalyst and a molar excess of formaldehyde. Novolak or novolac resins are prepared with an acid catalyst and less than one mole of formaldehyde per mole of phenol. The initial reaction involved in the preparation of resolated novolaks is carried out with an acid catalyst and less than a 1:1 mol ratio of formaldehyde to phenol. After formation of the novolak, the pH is adjusted so that the reacttion mixture is basic and additional formaldehyde is added. Resoles and resolated novolaks are inherently thermosetting and require no curing agent for advancement. Novolaks, by comparison, are thermoplastic and require the addtion of a curing agent, the most common being either hexamethylenetetramine or a resole. The stages of molecular weight advancement are characterized by liquid or solid phenolic polymer which is soluble in certain organic solvents and is fusible; a solid resin which is insoluble but swelled by organic solvents and, although softened by heat, exhibits essentially no flow; and an insoluble, infusible product which is not swelled by solvents nor softened by heat, i.e., the system is in a highly cross-linked state.
Phenolic resins have many uses. For example, such materials are used as bonding agents in friction materials such as brake linings, clutch facings, transmission bonds and the like. For example U.S. Pat. Nos. 4,096,108; 4,268,657; 4,218,361;, 4,219,452; and 3,966,670 describe various friction materials in which a phenolic resin is employed as the bonding agent. Phenolics are also used as molding materials, and as coatings and adhesives. Phenolics resins developed for non-flammability and long term temperature stability to 230.degree. C. have been studied in carbon-fiber composities. Potential for such composities lies in advanced aircraft application.
While present day phenolics exhibit several beneficial properties they suffer from a number of disadvantages which restrict their utility. For example, such materials exhibit less than desirable thermal oxidative stability. Other major problems of present day phenolic technology include a need for auxilary chemicals such as hexamethylene tetraamine to crosslink the phenolic which often results in the production of volatile by-products such as ammonia during crosslinking. Still other problems result from the fact that crosslinking is often extensive and is not controllable.
Various modifications to phenolics have been proposed to obviate certain of the disadvantages attendant to these resins. For example, epichlorohydrin has been reacted with the hydroxyl groups of novalak forming epoxy novalak. Moreover, n-chloro-2-propene has been reacted with the hydroxyl groups of novalak to form the corresponding form methylon resin. Similarly, Japanese patent publication Nos. 59-149918 and 58-34822 describe a method of preparing a phenolic resin containing cyanate groups. In this method, a trialkyl ammonium salt of a phenol novolak is reacted with excess cyanhalogenide in an organic solvent.
Various new polymers have been proposed. For example, T. M. Keller and J. R. Griffith "The Synthesis of a New Class of Polyphthalocyanine Resins," ACS Symposium Series, 132, "Resin for Aerospace", Clayton A. May, U.S. Pat. No. 4,304,896, and Arthur W. Snow and James R. Griffith, "Sythesis and Characterization of Heteroatom-Bridged metal-free Phthalocyanine Network Polymers and Model Compound" Macromolecules, Vol. 17 August 1984, pp. 1614-1624 describe a process in which cyano groups undergo chain extension polymerization to form a cyclic product known as a polyphthalocyanine.